SUMMARY (Project III) Over the past decade, it has become increasingly clear that Alzheimer's disease (AD) is a pathologically complex disorder that evolves over decades. Although the most common pathology of AD is the co-existence of amyloid plaques and neurofibrillary tangles, about 40% of AD cases also show a-synuclein (aS) pathology. It is also common to observe TDP-43 positive inclusions in AD cases. In dominantly inherited forms of AD (fAD), it has become clear that the deposition of A occurs well before the onset of cognitive symptoms and the appearance of tau pathology. Although the order of events may be less obvious in sporadic AD, the perception is that aS and tau pathologies occur as secondary events in the evolution of disease for fAD. A growing body of literature suggests that the evolution of intracellular aS and tau pathology may involve a prion-like spreading of a misfolded protein conformation along anatomical pathways or between cells in discrete anatomical structures. In studies preliminary to this application, we have developed models in which we can induce aS and endogenous tau pathology by exogenous seeding with aS. In Aim 1, we propose to use a combination of transgenesis and seeding to develop models that more faithfully recapitulate the various pathologies of AD. An important feature of these models is that, by seeding, we are able to establish point of origin and then track the spread of pathology to adjacent structures or anatomically connected structures. If misfolded proteins are moving between cells, as our data and data from other laboratories suggest, and if such proteins are exposed to the intercellular space for a significant interval of time, then antibodies directed against these proteins may be able to bind and inhibit further spread. The development of model systems that mimic the spread, or transmission, of human pathology offers an opportunity to test novel immune therapies to provide proof of concept for moving such therapies to humans. In Aim 2, we propose to use the unique features of these inducible models in proof of concept studies to determine the potential efficacy of antibody therapies. Given that a substantial proportion of AD cases have mixed pathologies, and that these pathologies are independently associated with neurodegeneration, we propose that to achieve optimal clinical benefit it is likely that therapies need to be developed that can target more than one type of protein inclusion pathology. Thus, in this project, we will develop and test immunotherapeutics targeting A, aS, and possibly tau. Collectively, these studies work towards two major unmet needs in AD, the generation of models that more faithfully reproduce human disease and development of disease modifying therapies.